The texture of a copy made from a diazo photosensitive paper tends to discolor into yellowish brown as a result of aging. This is mainly because the coupler particularly existing in the texture is oxidized by the oxygen in the air and is changed into a colored substance, said oxidation being remarkably promoted by ultraviolet rays.
The addition of an antioxidant, especially thiourea, to a photosensitive layer in order to prevent this discoloration has been known (J. Kosar, "Light Sensitive System", page 294), which is broadly adopted in preparation of an existing diazo photosensitive layers; however, the prevention of discoloration by adding such antioxidants is insufficient.
It is effective to cover the surface of a photosensitive layer with an ultraviolet absorber or to add said absorber into said layer for intercepting ultraviolet rays, which promote discoloration; but since the former requires an extra step, it is desirable to adopt the latter method. However, since almost all the effective ultraviolet absorbers are insoluble in water, it is not possible to add them to a general aqueous photosensitive liquid. In the case where aqueous dispersions of ultraviolet absorbers are used and then added to a photosensitive liquid, the dispersing agent tends to produce harmful effects on the photosensitive composition, causing lowering of the photosensitive effects, chromophoric properties as well as lowering of the color concentration and shelf life of the photosensitive paper. Also, the dispersing agent tends to cause hindrance, such as white spots and stripes, brought about by liquid repulsion, when the liquid is applied to the surface of a copy and further it is difficult to prepare a stable paint to be applied thereto.